30 Condition of the Moon's Surface. [January, 



has been radiated almost wholly away into space, and when 

 her oceans and atmosphere have disappeared through the 

 action of the same circumstances (whatever they may be) 

 which have caused the moon to be air-less and 'ocean-less. 

 But whether we take this view of our earth's future, or 

 whether we consider that her state has been from the begin- 

 ning very different from that of the moon, it nevertheless 

 remains probable that we see in our moon a globe which has 

 passed through a much greater proportion of its history (so 

 to speak) than our earth ; and accordingly the study of the 

 moon's condition seems capable of giving some degree of 

 information as to the future (possibly also as to the past) of 

 our earth. 



I wish, in the present paper, to consider the moon's con- 

 dition from a somewhat different point of view than has 

 commonly been adopted. It appears to me that the study 

 fo the moon's surface with the telescope, and the considera- 

 tion of the various phenomena which give evidence on the 

 question whether air or water exist anywhere upon or within 

 her, have not as yet led to any satisfactory inferences as to 

 her past history. We see the traces of tremendous sub- 

 lunarian disturbances (using the word " sublunarian," here 

 and elsewhere, to correspond to the word " subterranean " 

 used with reference to the earth), and we find some features 

 of resemblance between the effects of such disturbances 

 and those produced by the subterranean forces of our earth ; 

 but we find also as marked signs of distinction between the 

 features of the lunar and terrestrial crusts. Again, com- 

 paring the evidences of a lunar atmosphere with those 

 which we should expect if an atmosphere like our own sur- 

 rounded the moon, we are able to decide, with some degree 

 of confidence, that the moon has either no atmosphere or 

 one of very limited extent. But there our knowledge comes 

 to an end ; nor does it seem likely that, by any contrivances 

 man can devise, the further questions which suggest them- 

 selves respecting the moon's condition can be answered by 

 means of observation. 



But there are certain considerations respecting the moon's 

 past history which seem to me likely, if duly weighed, to 

 throw some light on the difficult problems presented by the 

 moon. 



In the first place, it is to be noted that the peculiar rela- 

 tion between the moon's rotation and revolution possesses a 

 meaning which has not hitherto, so far as I know, been 

 attended to. We know that now there is an absolutely 

 perfect agreement between the moon's rotation and revolu- 



